With wide use of an optical film for a display device, particularly for a liquid crystal display device, the optical film has been required to have higher functions. As a particularly important one of these requirements, an optical film having higher retardation for a longer wavelength in a visible light area is required. Such an optical film has a characteristic opposite to wavelength dispersion of an optical film constituted of a single plate of generally used resin such as polycarbonate, and such an optical film is therefore generally referred to as “reverse wavelength dispersion film”. In a reflective liquid crystal display device and the like, the reverse wavelength dispersion film can be used as a retardation film for converting linearly polarized light into circularly polarized light and for converting circularly polarized light into linearly polarized light. Recently, the reverse wavelength dispersion film is expected as a film whose object is to reduce color shift caused by a viewing angle of a polarization plate, that is, as a compensation film for polarization plate, and is further expected also as a protection film for polarizer, which has a retardation. As a single reverse wavelength dispersion film, a film containing cellulose acetate is known (for example, see Patent Document 1). However, due to the poor dissolubility of cellulose acetate to a solvent, available solvents for solvent casting method are limited. Further, cellulose acetate is difficult to be dissolved in a solvent of methylene chloride that is generally used in the solvent casting method. Further, as a cellulose acetate film is not easy to be processed and is hard to handle, an excessive amount of plasticizer is often used. However, when the plasticizer is used in an excessive amount, a problem arises in that the reverse wavelength dispersion decreases.
As a method for solving the problem caused by cellulose acetate, a reverse wavelength dispersion film made of cellulose acetate propionate is proposed (for example, see Patent Document 2).
While, the reverse wavelength dispersion film is expected as a retardation film for a reflective liquid crystal display device frequently used in the open air or a polarization plate compensation film for a large screen liquid crystal display device. Further, with wider variety of purposes of the liquid crystal display device, the reverse wavelength dispersion film has been used in various circumstances. It is preferable that environmental change, particularly temperature change hardly causes a crack to occur in the reverse wavelength dispersion film. However, the retardation film disclosed by Patent Document 2 has a problem in terms of durability in view of mechanical strength against temperature change.